The invention discloses an application of mercuric oxide. The mercuric oxide is taken as a photocatalyst to degrade an organic dye pollutant, so that the application scope of the mercuric oxide is wooden. The activity of the organic dye pollutant degraded by the mercuric oxide photocatalyst, such as a Rhodamine B dye is obviously higher than that of titanium dioxide P25 and is also obviously higher than that of a high-activity Ag3PO4 photocatalyst. Due to high durability of the mercuric oxide, the activity is not reduced after the mercuric oxide is recycled for more than 3 times. The mercuric oxide photocatalyst disclosed by the invention has high activity, high durability, simple preparation method and mild preparation conditions and facilitates popularization and application.